On a surface of an optical disk such as a CD (compact disk) or a DVD (digital versatile disk), concaves and convexes called as pits are provided. An optical head device is a device for radiating a laser beam to an optical disk and detecting light reflected from the disk to read an information recorded in the pits.
For example, linearly polarized light emitted from a light source is transmitted through a beam splitter, a collimator lens, a phase difference plate and an objective lens to reach an information recording plane of an optical disk. In this outgoing path, the linearly polarized light is straightly transmitted through the beam splitter and transformed into circularly polarized light by the phase difference plate. The circularly polarized light is reflected by the information recording plane of the optical disk to be circularly polarized light in the reverse direction, and is transmitted through the objective lens, the phase difference plate and the collimator lens in the returning path in the reverse order to the order of the outgoing path. In the returning path, the light is transformed by the phase difference plate into linearly polarized light polarized in a direction perpendicular to that of incident light. Accordingly, light in the returning path is linearly polarized in a direction 90° different from that of the light in the outgoing path, whereby the propagation direction of the light is turned by 90° by the beam splitter and the light reach a photodetector.
In the optical head device, if e.g. fluctuation of tilt of the optical disk occurs, the focal position of beam spot deviates from the recording surface. Accordingly, a servo mechanism for detecting and compensating such a deviation to make the beam spot follow the concave/convex pits in the recording surface. Such a mechanism is configured to adjust the focus of the beam spot emitted from a laser light source on the recording surface to detect a tracking position, so that the beam spot follows an objective track. Further, in the optical head device, it is necessary to prevent a laser beam reflected by the recording surface without hitting the pits from returning to the light source.
For these reasons, the optical head device requires an optical element for modulating (polarizing, diffracting, phase-adjusting, etc.) the laser beam from the light source. For example, the above phase difference plate has a function of effecting different refractive index depending on the angle between the optical axis of the phase difference plate and the phase plane of incident light, and shifting the phases of the two components of light produced by birefringence. The two light components having phases shifted from each other are synthesized when the light is output from the phase difference plate. The magnitude of the shift of the phase is determined by the thickness of the phase difference plate. Accordingly, by adjusting the thickness, a quarter wavelength plate for shifting the phase by π/2, a half wavelength plate for shifting the phase by π, etc. can be produced. For example, linearly polarized light passed through a quarter wavelength plate becomes circularly polarized light, and linearly polarized light passed through a half wavelength plate becomes linearly polarized light having a polarization plane tilted by 90°. By using such a characteristic and combining a plurality of optical elements, the above servo mechanism can be constructed. Further, the above optical element is employed also for preventing a laser beam reflected by the recording surface without hitting pits from returning to the light source.
The above optical element can be produced by employing a liquid crystal material. For example, a liquid crystal molecule having a polymerizable functional group has both a characteristic of polymerizable monomer and a characteristic of liquid crystal. Accordingly, when such liquid crystal molecules each having a polymerizable functional group are aligned and polymerized, an optically anisotropic material wherein alignment of the liquid crystal molecules are fixed can be obtained. Such an optically anisotropic material has an optical anisotropy such as a refractive index anisotropy derivable from a mesogenic structure, and by using this characteristic, a diffraction element or a phase difference plate is produced. As such an optically anisotropic material, for example, Patent Document 1 discloses a polymer liquid crystal obtained by polymerizing a liquid crystal composition containing a compound represented by CH2═CH—COO-Ph-OCO-Cy-Z (Z: alkyl group).
By the way, the above optical element is commonly required to have the following characteristics.
1) The optical element has an appropriate retardation value (Rd value) depending on wavelength to be used and application of the element.
2) Optical characteristics (Rd value, transmittance, etc.) are uniform in the entire surface of the optical element.
3) There is little scattering or absorption at the wavelength to be used.
4) Optical characteristics of the optical element can be easily adjusted to those of other materials constituting the element.
5) Wavelength dispersion of the refractive index or the refractive index anisotropy is small at wavelength to be used.
Particularly, it is important to have a proper Rd value indicated in item 1). Here, Rd value is a value defined by a formula Rd=Δn×d where Δn is a refractive index anisotropy and d is the thickness of the optical element in the propagation direction of light. In order to obtain a desired Rd value, if Δn of a liquid crystal material forming the optical element is small, it is necessary to increase the thickness d. However, if the thickness d increases, it becomes difficult to align the liquid crystal molecules, whereby it becomes difficult to obtain a desired optical characteristic. On the other hand, if Δn is large, it is necessary to decrease the thickness d, and in this case, it becomes difficult to precisely control the thickness. Accordingly, it is extremely important for such a liquid crystal material to have a proper Δn value.
In recent years, in order to increase the capacity of optical disks, use of laser beam having a shorter wavelength for writing or reading of an information has been in progress to reduce the concave/convex pit size of optical disks. For example, a laser beam having a wavelength of 780 nm is used for CDs, a laser beam having a wavelength of 650 nm is used for DVDs, and a laser beam having a wavelength of 405 nm is used for BDs (Blu ray Disk (product name)).
In next-generation recording media, still shorter wavelength may be used, and use of a laser beam (hereinafter it is also referred to as blue laser beam) having a wavelength of from 300 to 450 nm, tends to increase from now on. However, the optically anisotropic material described in Patent Document 1 is insufficient in the durability against a blue laser beam.
For example, when a phase difference plate prepared by employing such a liquid crystal is disposed in an optical head device using a blue laser beam as a light source, there occurs generation of aberration, decrease of the transmittance or change of the Rd value in the lapse of time in some cases. This is considered to be because the material of the phase different plate is damaged by exposure to the blue laser beam. If such an aberration is generated, light (light flux) emitted from the light source and transmitted through a collimator lens, a phase difference plate and an objective lens, cannot be focused into a point when it reaches a surface of a recording medium. As a result, light-utilization efficiency decreases and efficiency of reading or writing of an information decreases. Further, when the transmittance decreases, the intensity of light reaching the surface of the recording medium or photodetector becomes low, and in the same manner as above, the efficiency of reading or writing of an information decreases. Further, when the Rd value changes, for example, in a wavelength plate, it is not possible to maintain a desired ellipticity or the extinction ratio of linearly polarized light. As a result, the optical device may not function as an optical head device.
By the way, in order to reduce the size and increase the efficiency of an optical element, it is usually necessary to use a material having a high refractive index anisotropy. In general, a material having a high refractive index anisotropy has a high refractive index. However, since such a high refractive index material has a large wavelength dispersion of refractive index, such a material tends to have a high absorption of short wavelength light (that is, such a material has a high molar extinction coefficient). Accordingly, conventional high refractive index materials have a problem that they have low durability against short wavelength light such as a blue laser beam.
In order to improve durability against light, it is preferred to employ a material having a low molar extinction coefficient such as a compound having a complete alicyclic structure containing no aromatic ring. However, a complete alicyclic liquid crystal monomer usually has a small Δn, and there are problems that a polymer obtained from such a monomer has a further small Δn or such a polymer becomes isotropic, whereby it becomes difficult to obtain a desired liquid crystallinity.
For example, the following two complete alicyclic liquid crystal monomers exhibit optical anisotropy (birefringence), but form an isotropic polymer by polymerization.CH2═CH—COO-Cy-Cy-C3H7 CH2═CH—COO-Cy-Cy-C5H11 
For this reason, it is necessary to mix each of these monomers with another compound to form an anisotropic polymer. However, since the temperature range in which the above monomers show optical anisotropy is not wide, it is difficult to form a composition having a desired liquid crystallinity even if each of the monomers are mixed with another compound.